This invention relates to a camera which uses a distance detecting sensor for automatic focusing (AF) also as a light detecting sensor for automatic exposure (AE) and calculates a spot light detection information in the center of a viewfinder frame using an output of the light detecting sensor.
For example, in the case that a main object image and a background image coexist within a light detection area because a main object is at a long distance from a camera and the size of the main object image located in the center of a viewfinder screen is small, it is desirable to accurately discriminate a brightness balance of the main object and the background and to obtain a light detection information for the main object (in-focus position) with high accuracy in order to set a proper exposure control value for the main object. The same can also be said for cameras provided with a zoom lens because, in such cameras, the size of the main object image within the viewfinder screen varies according to a zoom ratio of the zoom lens regardless of an object distance and, accordingly, a proper exposure control value for the main object largely varies according to an angle of view. Since standard compact cameras of recent years are provided with a zoom lens, it is desirable to obtain a precise light detection information for the main object according to an angle of view with high accuracy.
There is known a camera which uses a distance detecting sensor for AF to perform a spot light detection in the center of a viewfinder screen in order to accurately detect a brightness of a main object image located in the center of the viewfinder screen. For example, Japanese Unexamined Utility Model Publication No. 55-97723 discloses a camera in which a photoreceptor for receiving a light reflected by a sensing surface of a distance detecting sensor is disposed in the vicinity of the distance detecting sensor, and a deviation of an output of the distance detecting sensor from an accurate brightness of an object is corrected by an output of the photoreceptor. Further, Japanese Unexamined Utility Model Publication No. 58-131023 discloses a camera in which, after an analog signal output from a distance detecting sensor is converted into a digital signal, a distance detection information and a light detection information are calculated.
However, in the cameras disclosed in the above publications, since the light detection information is obtained for an entire light detecting area of the distance detecting sensor without dividing it into sub-areas, it is difficult to obtain a highly accurate light detection information for an in-focus position (spot light detection information).
As a method for overcoming the above problem, Japanese Unexamined Utility Model Publication No. 3-35530 discloses a technique of dividing a light detecting area of the light detecting sensor into a plurality of sub-areas and calculating a light detection information for each sub-area. Further, for example, compact cameras adopt a technique of dividing a viewfinder screen into a central portion and a peripheral portion, collectively discriminating pieces of light detection information calculated for the respective sub-areas and determining a light detection information for AE. There is also known a technique of dividing a distance detecting area of a distance detecting sensor into a plurality of sub-areas and calculating a distance detection information for each sub-area.
In the case that the distance detecting sensor is used also as the light detecting sensor, it is preferable to divide the distance detecting area into a plurality of sub-areas and to detect a distance detection data and a light detection data for each sub-area. In such a case, since different calculations are performed to obtain an object brightness and an object distance using pixel data from the distance detecting sensor, it is necessary to effectively use the pixel data in consideration of calculation times, a memory capacity, other factors. More preferably, the distance detection calculation and the light detection calculation may be performed independently. However, such a method necessitates a memory for storing the pixel data for the respective calculations, leading to an increase in memory capacity. Particularly, if a method for dividing an image sensing area of the distance detecting sensor into a plurality of light detecting areas and distance detecting areas and calculating a distance detection information and a light detection information for each distance detecting area and for each light detecting area is adopted to enhance distance and light detecting accuracies, the calculations become more complicated, making it exceedingly difficult to reduce a memory capacity.
There is another problem resulting from the fact that the spectral sensitivity of the distance detecting sensor is generally skewed toward an infrared spectrum. Since the light detection information obtained based on the output of the distance detecting sensor differs from the one obtained based on a usual light detecting sensor whose spectral sensitivity lies in a visible spectrum, the use of the output of the distance detecting sensor directly as a light detection information is problematic in view of light detecting accuracy. Particularly, in the case that a light detecting sensor for obtaining an average light detection information for the entire viewfinder screen is separately provided and a control exposure value is set based on an average light detection data obtained from the output of the average light detecting sensor and the spot light detection data obtained from the output of the distance detecting sensor, a light detection level needs to be adjusted.
There is known a light detecting device which enhances a light detecting accuracy by correcting a light detection information calculated based on an output of a distance detecting sensor. For example, Japanese Unexamined Patent Publication No. 3-1030727 discloses a light detecting device which enhances the accuracy of a light detection information by correcting a light detection information obtained based on an output of a distance detecting sensor by a correction data for an object brightness obtained in the same position as a distance detecting area of the distance detecting sensor. More specifically, a light detecting element for monitoring the object brightness by receiving a light reflected by a sensing surface of a distance detecting sensor is provided in the vicinity of this sensing surface; a distance detection information is calculated using the output of the distance detecting sensor; a spot light detection information is calculated; and the calculated spot light detection information is corrected by the correction data for the object brightness which is calculated from the output of the light detecting element.
However, since the light detecting sensor for monitoring the object brightness is provided in the vicinity of the distance detecting sensor in the light detecting device disclosed in Japanese Unexamined Patent Publication No. 3-1030727, the distance detecting sensor used also for the spot light detection disadvantageously becomes larger. There may be adopted a method for correcting the output of the distance detecting sensor by providing an IR-cut filter directly on the distance detecting sensor. However, the adoption of such a method brings about problems: (1) the distance detecting sensor cannot play its principal role because an incident light is reflected by the IR-cut filter, and (2) an increased number of members lead to an increased production cost. In view of the above, it is preferred that the distance detecting sensor be used also as the light detecting sensor without changing the construction thereof.
Further, in the case that the distance detecting sensor is used also as the light detecting sensor, if a back light discrimination is made based on a brightness difference between the average light detection data and the spot light detection data, the spectral sensitivity of the distance detecting sensor differs from that of the light detecting sensor. If a light source illuminating the object is an artificial light source such as a fluorescent lamp or an incandenscent lamp, the spot light detection data has a reduced reliability. Accordingly, the discrimination result also has a reduced reliability. For example, since light from a fluorescent lamp contains almost no wavelength components longer than 700 nm, the spot light detection data is a value at an underexposure side while the average light detection data is a value at an overexposure side. Further, since light from an incandenscent lamp contains a great number of wavelength components within an infrared spectrum, the spot light detection data is a value at the overexposure side while the average light detection data is a value at the underexposure side. Thus, if the light source is an artificial light source, the brightness difference between the light detection data obtained by the light detecting sensor and the spot light detection data largely varies. It is difficult to make an accurate back light discrimination under an artificial light source and, therefore, the exposure control performed based on such an back light discrimination is highly likely to be proper.
Japanese Unexamined Patent Publication No. 3-253829 discloses a camera capable of discriminating whether or not a scene to be photographed is back lit based on a light detection data from a light detecting device and a spot light detection data from a distance detecting device. Further, Japanese Unexamined Patent Publication No. 60-129732 discloses a camera capable of discriminating whether or not a scene to be photographed is back lit based on a light detection data from a light detecting device and a spot light detection data from a distance detecting device, discriminating whether or not an object distance lies within a flash photographing permitting range based on the light detection data from the distance detecting device, and automatically firing a flash device to perform a daytime synchronized photographing if the scene to be photographed is back lit and the object distance lies within the flash photographing permitting range.
However, the cameras disclosed in the above two publications do not perform an exposure control in view of a variation in reliability of the light detection data depending on the light source in the case that the distance detecting sensor is used also as the light detecting sensor. Accordingly, it is difficult to perform a proper exposure control under an artificial light source.